Cushions for a chair are used on chairs such as, for example, office chairs. Well-known cushions used on the office chairs and the like include zabutons (i.e., seat cushion) that can be mounted on a seating face of a chair, and backrest cushions that are attached to a backrest member of a chair and abut on the back of a user seated.
However, the zabutons and the backrest cushions tend to result in a situation in which a part of buttocks of the user seated bear the body weight of the user, leading to compression of the buttocks and/or poor posture. In addition, the backrest cushions may bring the user into a feeling of compression of the chest or the abdomen and into difficulty in breathing.
Examples of well-known seat devices having a function of retaining a person's seated position that are principally applied to driver's seats for vehicles include a seat device having a function of retaining a person's seated position as disclosed in Japanese Unexamined Patent Application, Publication No. 2012-46005. The seat device as disclosed in the publication includes, in the vicinity of a position at which an ilium of a user seated is to abut, on the backrest member being to abut on the back of the user seated, a pair of ilium support projections which are fitted to ilium projections of the user. The seat device having a function of retaining a person's seated position prevents, by means of the ilium projections, the person seated while driving from being brought into a submarine state (a state in which the buttocks are moved forward and the pelvis leans rearward), whereby the user seated can easily retain his or her posture, leading to preclusion of occurrence of difficulty in breathing. Although the seat device having a function of retaining a person's seated position can be suitably used principally in the driver's seats for vehicles as mentioned above, the seat device does not always have a structure suitable for chairs such as office chairs.